The present invention relates to control systems for steam generators and the like and more particularly to control systems for boilers employed in electric power plants.
Various kinds of fuels can be used in the operation of power plant and other boilers, and some of these fuels such as coal, waste gases or other solid fuels can vary significantly in heating value. Other fuels such as natural gas and oil exhibit little variance in heating value.
The operation of a boiler is affected by fuel heating value changes since at a fixed flow an increase or decrease in fuel heating value results in increased or decreased boiler heat input rate and ultimately increased or decreased boiler heat output rate. For example, if the pulverized coal feed rate is set at a particular value and the BTU content of the coal drops, the boiler outlet steam will ultimately drop in pressure and temperature. In most boiler controls, a change in outlet steam conditions results in corrective fuel inflow which causes the steam conditions ultimately to return to desired values. The present invention is related to an improved arrangement in which corrections are made in boiler operations as a result of fuel heating value changes.
In electric power plants, it has long been common to control fuel input to hold outlet steam pressure from boilers at a regulated value and independently to compare inlet air flow to outlet steam flow and operate the fans to make corrective inlet air flow changes. During load changes, this "steam flow/air flow" system results in overfiring on load increases and underfiring on load drops. After a disturbance occurs in outlet steam conditions because of a fuel heating value change the control ultimately operates the boiler to correct the steam conditions in the steadystate. However, improper fuel/air balance can ressult in inefficiency. For example, the plant may be increased to maximum allowed air flow so that further load increase is not permitted yet maximum load would not have been reached because fuel has not been increased in balance with the inlet air increase.
Provisions have been made in the prior art for adjusting boiler operations when changes occur in fuel heating value, but so far as is known such provisions have been limited to steam flow/air flow type systems in which process transient response to control actions has been generally poor. U.S. Pat. No. 2,328,498 exemplifies this approach.
In the more recent parallel type of boiler control system encouraged by the increased use of once-through boilers, input fuel and air are both controlled in response to outlet steam flow to provide good steady state response and fast and smooth transient response to load changes. Further, oxygen detection has been used in the parallel type of control to adjust air flow as changes occur in the rate at which burnable fuel enters the combustion zone, and as a result some correction does occur in air flow control for changes in fuel heating value. However, to prevent smoking or more generally to hold the fuel and air in proper balance, the corrections are made only the fuel/air balance based on a signal corresponding to the existing input fuel flow rate which is incorrect to hold desired outlet steam conditions at the existing load because its heating value has changed. This approach is not entirely adequate because it involves excessive process transient behavior in the course of achieving process corrections for fuel heating value changes. Thus, a steam pressure upset is always followed by a fuel/air balance upset and load changes are always accompanied by fuel transients if the fuel heating value has changed from the value to which the control system is tuned.
One other prior art approach involving an adjustment effect for fuel heating value variation in the parallel type boiler control is one in which steam flow and drum pressure rate of change are used to develop a heat release signal. A high select is then made on the heat release signal and a signal indicative of the mass input fuel flow. As a result, the system functions only on high select and accordingly is useful principally to prevent boiler smoking on load pickups or on the sudden inflow of a richer fuel. If a poorer fuel begins to be used, the system has no direct response because of the high select arrangement. Further, with the use of drum pressure rate of change, the system is responsive only to load transients or more or less step changes in the heating value of input fuel. Fuel heating value changes most often occur over long time periods such as several days, and the system using drum pressure change rate is accordingly not responsive to provide direct corrective action for changes in fuel heating value under most circumstances.
It has also been the practice in some cases to obtain a fuel sample and determine its heating value with the use of an off-line calorimeter. The plant operator subsequently makes a control system adjustment in accordance with the sampling results, and the plant is then tuned and can be properly operated. However, this approach does not provide continuous adjustment.
To provide continuous control adjustment for changes in fuel heating value, it might be desirable to employ a device which can directly and continuously sense fuel heating value and generate a signal representative of it. However, no such device is known to be available for commercial applications. Therefore, in providing boiler operating corrections in response to full heating value changes, the present invention employs signals representative of conditions which are inferentially related to fuel heating value. The cross-referenced application is directed to certain basic and specific aspects of the invention and the present application is directed to an improvement embodiment.